The objective of this project is to understand the role that the adhesion molecule N-cadherin plays in maintaining hematopoietic stem and progenitor cells. All of the cells in the blood descend from hematopoietic stem cells (HSCs), which reside in a complex microenvironment within the bone marrow. Proper regulation of this "HSC niche" Is believed to be important for maintaining HSC homeostasis, and alterations to bone marrow stromal cells have been shown to contribute to the development of both leukemia and bone marrow failure syndromes. Moreover, pharmacologic manipulation of the niche can result in the mobilization of both HSCs and progenitors from the marrow into the peripheral blood, which is used clinically to collect HSCs for therapeutic use. One putative component of the niche is the adhesion molecule N-cadherin, which may maintain HSCs through homotypic osteoblast-HSC interactions. However, controversy abounds as to the role of N-cadherin in HSC regulation, with some groups calling into question whether HSCs even express the molecule. In order to address this problem, we will conditionally delete N- cadherin from hematopoietic cells and osteoblasts. In Aim 1, we will define the role of both stromal and hematopoietic N-cadherin in supporting HSC function. HSC number and cell cycle status in the bone marrow will be quantified, and HSC self-renewal and long-term repopulating activity will be assessed using competitive bone marrow transplantation and serial bone marrow transplantation. In Aim 2, we will determine if N-cadherin plays a role in HSC and progenitor mobilization using the canonical mobilization agent, G-CSF. The number of hematopoietic stem/progenitors in the blood, spleen, and bone marrow at baseline and after G-CSF will be determined. To determine if N-cadherin deficient cells are more easily mobilized, mixed chimeras of wild type and N-cadherin null bone marrow will be mobilized with G-CSF. RELEVANCE Hematopoietic stem cells make all the blood in the human body and are the only stem cells currently used therapeutically. Alterations in hematopoietic stem cells can result in diseases such as leukemia or bone marrow failure. By further understanding how hematopoietic stem cells are regulated by the cells which surround them, we hope to better understand the pathogenesis of hematological diseases.